This invention relates to high performance stepper motors of the linear or rotary type which may be used in various applications including the electronic typewriter.
Variable reluctance stepper motors have been conveniently used as incremental motion transducers. However, for applications requiring a high force-to-mass ratio, such stepper motors have not, in some instances, been used because the force developed per unit of moving mass is not as high as for certain DC motors including both commutator type rotary DC motors and voice-coil linear DC motors.
Fredrickson U.S. Pat. No. 3,292,065 discloses a variable reluctance stepper motor of the linear type. In the embodiment of Fredrickson which achieves the highest force-to-mass ratio, the stator means is excited at the various pole positions on both sides of the longitudinally extended stator-to-stator air gap, i.e., double-sided excitation. However, the force-to-mass ratio of Fredrickson suffers for two reasons. First, the Fredrickson structure permits flux leakage longitudinally through the slider. In this connection, it will be noted that the nonmagnetic discontinuities between the teeth of the slider are relatively shallow, i.e., these discontinuities do not extend perpendicular or transverse to the direction of movement of the slider a distance substantially greater than the space between the teeth. Moreover, there are no additional discontinuities of nonmagnetic material in the slider. Accordingly, a rather unlimited longitudinal flux path through the magnetic material of the slider may be established for flux leakage which reduces the force-to-mass ratio for the slider of the motor. Second, the slider of Fredrickson extends outwardly beyond the ends of the stator so that a substantial portion of the slider is not generating force to drive the slider. Moreover, the slider portion extending beyond the air gap creates end effects which are detrimental to the drive. It will also be noted that the highest force-to-mass ratio embodiments disclosed in the Fredrickson patent require stator means with winding or excitation means on both sides of the longitudinally extending air gap which receives the slider.
Chai U.S. Pat. No. 3,867,676 also discloses a variable reluctance stepper motor of the linear type with double-sided excitation. Chai demonstrates no apparent concern for longitudinal flux leakage or its adverse effects on force-to-mass ratio. Significantly, the minimum thickness of the Chai slider which provides a longitudinal flux leakage path is always substantial relative to the maximum thickness at the extremities of the teeth, e.g., the minimum thickness is at least 25% of the maximum thickness. There is no discussion of an effort to minimize longitudinal flux leakage nor is there any suggestion in the specification that such longitudinal flux leakage has been minimized. In FIG. 9 where Chai achieves the minimum longitudinal flux leakage since the thickness of the slider is substantially less than the maximum thickness, the force-to-mass ratio is particularly small. This small force-to-mass ratio is the result of a single tooth per stator pole and a slider which always includes a substantial portion which extends beyond the stator structure and thereby produces no force.
Similarly, Schreiber et al. U.S. Pat. No. 3,162,796 also discloses a variable reluctance stepper motor of the linear type but demonstrates no interest in achieving a high force-to-mass ratio. In all of the Schreiber et al embodiments, there is a single tooth per stator pole and the slider extends beyond the stator structure so as to produce a small force-to-mass ratio. In almost all of the Schreiber et al embodiments, there is no stator structure on the interior of the cylindrical slider, and the minimum thickness of the slider is almost as great as the maximum thickness so as to permit return of the flux longitudinally through the slider which necessarily reduces the force-to-mass ratio. The embodiment of FIGS. 14 and 15 does disclose the use of interior stator structure which permits a "reduction in weight" of the slider although there is no suggestion that the force-to-mass ratio is increased and the shape of the teeth which preclude any effective generation of force by the exterior stator structure in FIG. 14 and the interior stator structure in FIG. 14 suggest a low force-to-mass ratio. In connection with FIG. 15, there is the suggestion that the portion of the slider between the teeth may even comprise a non-magnetizable material. However, there is no suggestion that the non-magnetizable material is chosen for purposes of limiting longitudinal flux leakage and the suggestion that the material be "austenitic boron steel" precludes a further reduction in weight.
An article entitled Characteristics of a Synchronous Inductor Motor, Snowdon and Madsen, Trans, AIEE (Applications in Industry) vol. 8, pp. 1-5, March 1962, discloses a stepper motor having a rotor which is confined to the air gap of the stator. However, the force-to-mass ratio is relatively small since the rotor acts as a longitudinal flux return path to a single-sided stator. In order to provide this longitudinal flux return path, the minimum thickness of the rotor between the teeth of the rotor is substantial relative to the maximum thickness of the rotor at the teeth. An article entitled A Self-Oscillating Induction Motor for Shuttle Propulsion, Laithwaite and Lawrenson, Proc. IEE, vol 104, part A, No. 14, April 1957, suggests that the rotor of Snowdon and Madsen might be unwound. However, the resulting slider would still have to provide a longitudinal flux return path for a single-sided slider. Therefore, even if the slider were shortened as disclosed in an article entitled Linear Induction Motors, Laithwaite, IEE, paper No. 2433 u, Dec. 1957, the slider would still have a relatively low force-to-mass ratio and while this force-to-mass ratio might be increased by utilizing the double sided stator disclosed in Linear Induction Motors, the configuration of the slider with its longitudinal flux leakage still severely limits the force-to-mass ratio.